This invention relates to a structuring layer of a structuring belt used for structuring a wet fibrous web by means of pressing in a press section of a tissue papermaking machine for manufacturing high bulk tissue paper, wherein the structuring layer has a web-carrying side with a surface for cooperating with the fibrous web, said surface having depressions forming a three-dimensional structure of the surface. The invention also relates to a structuring belt for structuring a wet fibrous web in a press section by means of pressing in a tissue papermaking machine for manufacturing high bulk tissue paper. The invention further relates to a press section employing such a structuring belt, and to a tissue papermaking machine having such a press section.
The invention also relates to a method of manufacturing a structured high bulk tissue paper web and to such a high bulk tissue web.
The invention relates furthermore to a method of converting or upgrading an existing tissue papermaking machine.
The term “tissue paper” as used herein refers to soft paper with a basis weight usually of less than 25 g/m2. Tissue paper web having a basis weight of 10-50 g/m2 (more preferably 15-25 g/m2) forms a base paper from which certain single-ply and/or multi-ply products (e.g., napkins, towels, and toilet paper) can be manufactured. The term “high bulk” tissue paper means that the bulk is about 8-20 cm3/g and the single-ply tissue paper web thickness is about 160-400 μm.
Tissue paper is manufactured from a mixture of hardwood and softwood cellulose fibers, usually from a so-called “virgin” pulp constituting fresh fibers, as opposed to recycled fibers. Alternatively, recycled fibers can be mixed in with virgin fibers to make the pulp. Depending on the particular products to be made, various mixtures of fibers can be used. For example for toilet paper and/or facial tissue, the pulp may comprise 50-90% by weight of hardwood and 50-10% by weight of softwood fibers, a preferred mixture being about 70% hardwood and 30% softwood. For paper towels, the pulp may comprise 0-50% by weight hardwood and 100-50% by weight softwood fibers.
In manufacturing creped tissue paper, typically the formed wet paper web of cellulose fibers is dewatered before final drying on a Yankee cylinder, the dewatering usually being performed by either a pressing technique or a through-air-drying (TAD) technique. In some conventional tissue machines a suction pressure roll or a blind-drilled roll is used as a press roll that presses the web against the Yankee cylinder, but this compression of the web results in a final tissue product with relatively low thickness and low bulk such as 5-9 cm3/g. In other conventional tissue machines the web is pressed and dewatered in a double-felted pre-press before the reaching the Yankee cylinder, the pre-press being formed by two press rolls that define a press nip therebetween, but again, the rolls compress the paper web uniformly and it results in the web having relatively low thickness and bulk. It has been proposed to use an extended-nip press such as a shoe press as an alternative to the above-mentioned conventional pressing techniques, which extended- or long-nip press can apply lower pressure but provides a longer dwell time in the nip. As another alternative, it has been proposed to use a shoe press against the Yankee cylinder in order to decrease the compression of the web in the press nip, so as to increase the bulk or thickness of the web. The objective has been to achieve the same level of bulk or thickness as achieved by the TAD technique, but up to now this has not been possible. The thickness or bulk of the paper is important for the absorption ability of the paper and the feel of softness. The TAD technique therefore is still superior to the pressing technique in terms of achieving high bulk or thickness of the paper web, but it has the disadvantage that it necessarily requires higher energy consumption than is the case with a pressing technique, especially when TAD is used as a pre-drying process on a web containing a substantial amount of water to be removed. When TAD is used in place of the Yankee cylinder for final drying of the tissue web, the TAD technique requires less drying capacity than when it is used for pre-drying, and therefore has a lower energy consumption. Therefore, using TAD for final drying could be a viable alternative to the use of a Yankee cylinder.
In tissue papermaking machines that employ the pressing technique for dewatering the paper, a press felt runs together with the tissue web through the press nip, and the press felt receives water squeezed from the web and carries the water away. In order to achieve a high bulk, it is preferred to use only one press nip, but in some cases a single nip cannot achieve sufficient dewatering and hence one has to compromise and use a second press nip.
In tissue machines employing the pressing technique, it is also possible to use a “structuring clothing”, which is a clothing whose web-contacting surface has a lot of voids and top portions distributed between the voids. As the structuring clothing passes along with the tissue web through the press nip, the voids receive the fiber network of the tissue web, and therefore only those areas of the web contacted by the top portions of the clothing are compressed. Furthermore, in order to reduce compression in the nip and thereby increase the bulk or thickness of the fiber web in comparison with that obtainable using smooth roll presses, it is possible to use an extended-nip press as noted above. Such a structuring clothing can be a woven wire, belt, or fabric, including but not limited to a TAD fabric. The woven structure of such a fabric forms the voids and top portions of the web-contacting surface as described above. Such a fabric can further have a special coating (e.g., of a photo-sensitive resin or other type of material) to emphasize or form the pattern to be embossed into the wet tissue web during pressing in the press nip.
The term “structuring” of the paper as used herein refers to a process in which a three-dimensional pattern of the structuring layer of a structuring belt is embossed into the wet fibrous web during a pressing process when the fibrous network structure fills the three-dimensional (3D) pattern of the structuring belt layer, and in which fibers in the wet fibrous web are still movable relative to each other so that they are advantageously brought to new positions and directions relative to each other by the action of the elastically compressible press felt, which presses the wet fibrous web into the three-dimensional pattern or voids of the structuring belt layer, thereby promoting fiber binding between the fibers of the network and achieving partial dewatering of the wet fibrous web while achieving an increased bulk at the same basis weight, and MD and CD tensile strengths of the finally dried tissue paper web comparable to those of conventional tissue paper, and achieving an improved structure of this basic tissue paper.
U.S. Pat. Nos. 6,547,924 and 6,340,413 describe a tissue papermaking machine in which a structuring belt carries the fibrous web from the last press in the press section to the drying cylinder. However, the papermaking machine described in these references cannot produce a tissue paper of sufficiently high quality and high bulk while also achieving an acceptable dryness to make this machine concept commercially attractive/interesting. The described machines, because of the plurality of press nips required in order to meet the requirements for the dryness of the web for its runnability, do not meet the wishes of customers today to have a high bulk paper. Furthermore, there were problems with runnability of the machine, as either the web was too wet because the press felt was saturated with water and could not absorb a sufficient quantity in the nip, which led to paper breaks, or the dryness was sufficient and the runnability was good but the bulk or the quality of the final tissue paper web was too low. Additionally, the impermeable structuring belt described in these patents has a polymer web-contacting layer that includes grooves, but the dimensions of the grooves are such that the desired hydraulic pressure into the grooves cannot be created, which results in runnability problems, particularly web breaks as noted.